Including a semiresonant slotted mode stirrer

ABSTRACT

A microwave oven having a field stirrer comprising a circular sheet metal disc having a plurality of radial slots arranged about its periphery. The ends of the segments thus formed are preferably bent at different angles. The slots and segments together form semiresonant elements to which the energy from the microwave generator couples directly and then is reradiated by the rotating disc which acts as a moving virtual energy source. The disc is rotated so that the segments pass by the microwave energy coupling aperture for the oven. The oven combination produces a more uniform microwave energy distribution within the oven cavity and also improves the matching of the microwave generator to the cavity load so that the generator operates in the load region most suitable to the application.

United States Patent [1 1 Corcoran et al.

[ Mar. 18, 1975 INCLUDING A SEMIRESONANT SLOTTED MODE STIRRER [75]Inventors:- Richard P. Corcoran, Hempstead;

Alfred E. Hirtler, West Babylon; Robert F. MitteLLevittown, all of NY.

[73] Assignee: North America Philips Corporation,

New York, NY.

[22] Filed: Mar. 9, 1973 [21] Appl. No.: 339,539

52 us. Cl. 219/10.5 sn

[51] Int. Cl. H05b 9/06 [58] Field of Search 219/l0.55

[56] References Cited UNITED STATES PATENTS 3 626,l36 12/1971 Funahashi219/1055 3,716,687 2/1973 Constable .4 219/1055 Primary E.runzincrBruceA. Reynolds Attorney, Agent, or Firm-Frank R. Trifari; Bernard Franzblau[57] ABSTRACT A microwave oven having a field stirrer comprising acircular sheet metal disc having a plurality of radial slots arrangedabout its periphery. The ends of the segments thus formed are preferablybent at different angles. The slots and segments together formsemiresonant elements to which the energy from the microwave generatorcouples directly and then is reradiated by the rotating disc which actsas a moving virtual energy source. The disc is rotated so that thesegments pass by the microwave energy coupling aperture for the oven.The oven combination produces a more uniform microwave energydistribution within the oven cavity and also improves the matching ofthe microwave generator to the cavity load so that the generatoroperates in the load region most suitable to the application.

16 Claims, 10 Drawing Figures Pmmrmmmams 3872276 SHEETIEUFZY Fig. 3

6b" I 20b I Fig. 60

- ZIZOO INCLUDING A SEMIRESONANT SL OTTED MODE STIRRER BACKGROUND OF THEINVENTION This invention relates to a microwave oven and moreparticularly to a microwave oven having an improved field stirrer whichproduces a more uniform distribution of microwave energy within the ovenenclosure.

It is well known that microwave energy can be used successfully forheating or cooking various dielectric materials, especially for cookingfoodstuffs in a rapid and economical manner. The amount of heatdeveloped in the work material is a function of the intensity andfrequency of the electromagnetic field and the electrical properties ofthe material. A suitably dimensioned closed metal cavity into whichmicrowave energy can be introduced makes a good microwave oven.Microwave energy coupled into the cavity produces a pattern of standingwaves. A problem inherent in this type of oven is that the intensity ofthe electric field is not constant throughout the cavity. This may giverise to an unequal distribution of heat'within the work material. Inorder to overcome this problem, the cavity can be dimensioned for aso-called multi-mode operation and the field patterns within the cavitycan be periodically shifted in position so that a more uniformdistribution of heat is achieved in the work material. If the cavity isproperly dimensioned, a small change in the microwave generatorfrequency cancause a changeover to a different mode or field pattern inthe cavity. A change in the shape of the cavity, or a change in boundaryeffects, also can be used to switch modes in the cavity.

One commonly used device for effecting a change in the boundary effectsis a field stirrer. The stirrer is generally a propeller type fan havinga plurality of radially disposed metal fan blades which are rotated by amotor. The stirrer fan is mounted within the oven cavity so as tointercept a substantial portion of the microwave energy radiated fromthe generator and to deflect the energy impinging upon the fan bladestowards the oven load. The blade rotation influences the generatorfrequency and also changes the mode pattern within the cavity bychanging the boundary effects. One such field stirrer is described inUS. Pat. No. 2,8l3,l85. Although the use of known field stirrersimproves the en ergy distribution'within the cavity, there is still roomfor further improvement, especially in ovens designed to accomodate awide variety of load materials and of widely varying shape and size.

It is therefore an object of the present invention to provide amicrowave oven which will uniformly heat or cook foodstuffs or othersuitable load materials.

Another object of the invention is to provide a new and improved form offield stirrer for use in a microwave oven to produce a more uniformdistribution of the electromagnetic energy in the oven cavity.

A further object of the invention is to provide a microwave oven with afield stirrer that can be readily adjusted to match a wide variety ofoven loads.

SUMMARY OF THE INVENTION The microwave oven of the subject inventionincludes a novel field stirrer arrangement that comprises a circularsheet metal disc having a plurality of radially extending slots arrangedaround its periphery. The ends of the segments formed by the slots canbe bent at the same or different angles. The disc is mounted directly ona revolving metal or dielectric shaft or by means of dielectric spacerson a revolving shaft so that the segments pass in front of a couplingaperture in the oven by means of which the microwave energy isintroduced into the oven cavity.

As they revolve past the coupling aperture, the bent ends of thesegments formed by the slots reflect the microwave energy coming fromsaid aperture at different angles depending upon the degree of bend inthe different segments. The angle of bend also is used to perform asecondary function in that it influences the matching of the microwavegenerator, for example, a magnetron; to the load, i.e. the microwaveoven cavity.

The slots in the disc also serve a dual function in that when cut to theproper depth, the slots and the segments produced thereby formsemiresonant elements which couple to the microwave fields in the cavityand reradiate some ofthe energy in various directions in the ovencavity. The slots, or semiresonant elements, are analagous to shortcircuited sections of two wire transmission lines in which the metalsurrounding the slot constitutes the inductance and the opposite wallsof the slots act as the plates of a capacitor. Surface currents areinduced in the metal disc which causes coupling between the variousslots. By varying the degree of bend and the slot dimensions, therotating stirrer interacts with fields of different frequencies to agreater or lesser extent. The rotating stirrer thus provides an activefunction because of the direct interaction of the fields and thesemiresonant elements. At the same time, the slots and the segmentsformed thereby also affect the matching of the microwave generator tothe load. Since the disc is made of sheet metal, it is possible toreadily adjust or vary the number of slots, the width and the depth ofthe slots, the angle of bend in the ends of the segments, the positionofthe stirrer, etc. so that the microwave generator can be made tooperate in the most advantageous region (sink region) of its load(Reike) diagram, thus producing the greatest overall efficiency ofoperation for the entire microwave oven system. Alternately, themicrowave generator may be operated away from the sink region to gainadditional stability of operation in those applications where thegenerator is consistently lightly loaded.

The distribution of microwave energy to the load obtained with the ovenof the present invention has been found to be substantially uniform. Thestirrer used herein also serves to excite several modes in the oven. Byperiodically changing the field distribution or mode pattern inside theoven, the energy distribution is made uniform throughout the oventhereby producing uni form heating in all parts of the oven.

BRIEF DESCRIPTION OF THE DRAWING The foregoing and other objects andadvantages of the invention will become apparent from the followingdescription of a preferred embodiment of the invention taken inconnection with the accompanying drawing in which:

FIG. 1 illustrates a sectional view of a preferred embodiment of amicrowave oven according to the invention,

FIGS. 2 and 3 together illustrate the details of the new and improvedfield stirrer construction,

FIGS. 4A-7D are sectional view of the field stirrer of FIG. 2,

FIG, shows a portion of a second embodiment of the field stirrer, and

FIGS. 6A and 6B are sectional views of the stirrer of FIG. 5 that helpto illustrate the construction thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT I Referring to FIG. 1 of thedrawing, a hollow rectangular thin walled metallic enclosure or cavity 1is adapted to serve as a microwave oven for heating or cookingfoodstuffs, or the like. The dimensions of the oven are preferably largecompared with the wavelength of the microwave energy supplied by amagnetron 2 to the oven cavity. The microwave energy generated by themagnetron is coupled into the oven cavity by means of a waveguidesection 3 and a rectangular or other suitable coupling aperture 4 formedina metal wall 5. The microwave energy is supplied to the oven cavitythrough the back wall 6 of the oven.

A shelf 7 is secured to the oven walls and is located near thebottom'wall 8 of the oven. The shelf is made of any material that istransparent or semi-transparent to microwave energy and is adapted tosupport a heating load 9, e.g., an article of food to be cooked. Theshelf may be partially composed of a lossy dielectric material to insurethat the cavity has some load in the event that the oven is operatedwithout its normal load. This serves to protect the magnetron fromdamage due to excessive amounts of reflected microwave energy. In orderto gain access to the oven interior, an opening 10 is provided in thefront wall 11 of the oven. The opening 10 is covered by a hinged metaldoor 12 having means, not shown, for sealing the opening against theleakage of microwave energy when the door is closed. When the door isclosed, the oven cavity is entirely closed except for the couplingaperture 4 through which the-microwave heating energy enters the cavity.

A field stirrer 13, shown in greater detail in FIGS. 2-4, is mounted ona vertical extending shaft 14 made of a metal or a dielectric material.Theshaft 14 is mounted for rotationin a bearing 15 secured in the topwall 16 of the oven. The shaft and stirrer are rotated by .means of amotor 17 to which the shaft is coupled. Ob-

viously, suitable meansfor sealing the shaft opening in top wall16against energy leakage must be provided, such sealing techniques beingknown in the art The shaft and stirrer are preferably symmetricallylocated in the top wall 16 of the oven. As the stirrer is rotated by themotor the end segments or fins pass in front of the coupling aperture 4to provide the above described desirable"stirring action on themicrowave field within the oven cavity.

Turning now to FIGS. 2-4 there is shown, in detail, the novel fieldstirrer 13 of the microwave oven described with reference to FIG. 1. Thestirrer 13 is a thin circular sheet metal disc having a pluralityof-radially extending'slots l8 symmetrically arranged about theperiphery of the disc to form a plurality of segments 19.

The ends 20 of the segments can all be bent up or down at the sameangle. Preferably, the fins or segment ends 20 are bent at differentangles. In one embodiment of to bend all of the fins downward at thesame angle or at different angles. It is furthermore possible to bendsome ofthe fins up and others down. This feature of the invention isbest illustrated in FIGS. 5 and 6 wherein the end segment 20,, is bentup and the end segment 20,, is bent down. The optimum configuration ofthe fins will vary with the dimensions and shape of the various ovencavities. Since the stirrer is made of a thin sheet metal, the bestconfiguration of the fins can be readily adjusted by test or servicepersonnel for each individual oven. The height of the shaft 14 (FIG. 1)also can be adjusted to provide optimum performance of any given ovencavity. Furthermore, it is feasible to vary the number of slots and thewidth and the depth of the slots to provide optimum oven heatingperformance. As discussed above, the slots and segments form semiresonant elements to which some of the energy from the microwave generatorcouples directly, as well as coupling to fields in the oven cavity. Thiscoupled energy is then reradiated from the rotating stirrer which actsas a moving virtual source to improve the energy distribution in theoven cavity.

It will be clear from the above that the invention provides a largedegree ofdesign flexibility for the designer of microwave ovens. Inaddition, as discussed above, the invention produces a more uniformenergy distribution pattern in the oven cavity and improves the matchingof the microwave generator to the cavity load. Thus, the novel stirrermakes it possible to load the magnetron tube near its sink region, avery desirable feature from the viewpoint of overall system efficiencyvOne embodiment of the invention which was tested and found to producegood overall results had the following approximate dimensions. Thediameter of the stirrer disc was 13 inches and it was 0.04 inches thick.The radial distance from the center of the disc to the interior endofthe slot was 2 inches and to the fin it was 5 inches. The slot was 3 4inches long and 4 inch wide. The shaft 14 extended 4 of an inch downfrom the top wall 16 of the oven. The oven was l4 inches high, 15 inchesdeep and 16 inches wide. The coupling aperture was a rectangular opening1 inch by 4 inches symmetrically located at the top of the back wall andequally distant from the ovenside walls. The waveguide section was 2inches high, 4 inches wide and 6 inches long.

Although a particular embodiment of the invention has been describedabove it is to be understood that the invention is not limited to theparticular details so described, as many equivalents will suggestthemselves to those skilled in the art. Furthermore, many modificationsthereof will become apparent to those skilled in the art withoutdeparting from the spirit and scope of this invention. In view of theforegoing, we do not wish to be limited to the details of the inventiondisclosed herein except as defined by the appended claims.

We claim:

1. A microwave oven comprising a substantially closed metallic cavityadapted to hold an energy absorbing load, an aperture in a wall of saidoven for cou pling microwaveenergy into the oven cavity, means forgenerating and coupling microwave energy to said aperture, a field'stirrer rotatably mounted in the cavity and comprising a thin metal dischaving a plurality of radial slots arranged about its periphery to forma plurality of segments with the outer ends of at least some of saidsegments being bent out of the plane of the disc whereby said slotsand-se gments form semiresonant elements which couple to the R.F. fieldsin the oven and reradiate the energy in various'directions, and means,for rotating said disc so that the bent ends pass near i 4. A mircowaveoven as claimed in claim 2 wherein the bending angle of the end segmentsvaries progressively about the disc periphery.

5 A microwave oven as claimed in claim 1 wherein some of the discend'segments are inclined-upward from the plane of'the disc and otherones of said end segments are inclined downward from the plane of the 6.A microwave oven as-claimed in claim 1 wherein the disc is composed ofsheet metalof a thickness which allows it to be easily bent to'formdifferent adjustable angles of inclination to the plane of the disc.

7. A microwave oven as claimed in claim 1 wherein said aperture islocated in a side wall of the cavity near the top wall and said stirreris mounted on a vertical shaft extending from said cavity top wall.

8. A microwave oven as claimed in claim 1 wherein at least onediametrically opposed pair of end segments are inclined in the samedirection to and at the same angle to the plane of the disc.

9. A microwave oven comprising a substantially closed metallic cavityadapted to hold an energy absorbing load, means for supplying microwaveenergy into the oven cavity, a field stirrer rotatably mounted withinthe cavity and comprising a thin metal circular disc having a pluralityof radial slots therein to form rainteract. with the energy supplied tothe oven cavity thereby to produce a cyclically repetitive distributionof energy within the cavity which is substantially uniform in the regionof the cavity to be occupied by the load.

10. A microwave oven as claimed in claim 9 wherein said fins areuniformly disposed about the disc periphery and at least some of whichare inclined at different angles to the plane of the disc.

11. A microwave oven comprising a substantially closed metallic cavityadapted to hold an energy absorbing load, means for supplying microwaveenergy into the oven cavity, a field stirrer rotatably mounted in thecavity and comprising a thin metal circular disc having a plurality ofradial slots symmetrically arranged about its periphery to form aplurality of segmentswith the outer ends of at least some of saidsegments being bent out ofthe plane of the disc, said slots beingdimensioned so as to form semiresonant elements to which energy fromsaid energy supplying means is coupled directly and then is reradiatedto the oven cavity, and means for rotating said disc so as to interceptand interact with the energy from said energy supplying means and theenergy within the fields in the oven cavity thereby to improve theenergy distribution within said I the same angle to the plane of thedisc.

end segments are inclined downward from the plane of v the disc. g

16. A microwave oven as claimed in claim '11 wherein the width of theslots is relatively narrow with respect to the width of the segment areabetween adjacent slots in the plane of the disc.

1. A microwave oven comprising a substantially closed metallic cavityadapted to hold an energy absorbing load, an aperture in a wall of saidoven for coupling microwave energy into the oven cavity, means forgenerating and coupling microwave energy to said aperture, a fieldstirrer rotatably mounted in the cavity and comprising a thin metal dischaving a plurality of radial slots arranged about its periphery to forma plurality of segments with the outer ends of at least some of saidsegments being bent out of the plane of the disc whereby said slots andsegments form semiresonant elements which couple to the R.F. fields inthe oven and reradiate the energy in various directions, and means forrotating said disc so that the bent ends pass near said aperture tointercept and reflect the microwave energy entering through the apertureso as to periodically change the mode pattern within the oven cavity. 2.A microwave oven as claimed in claim 1 wherein the bent ends of saiddisc segments are inclined at different angles to the plane of the disc.3. A microwave oven as claimed in claim 2 wherein diametrically opposedpairs of end segments of the discs are inclined at the same angle to theplane of the disc.
 4. A mircowaVe oven as claimed in claim 2 wherein thebending angle of the end segments varies progressively about the discperiphery.
 5. A microwave oven as claimed in claim 1 wherein some of thedisc end segments are inclined upward from the plane of the disc andother ones of said end segments are inclined downward from the plane ofthe disc.
 6. A microwave oven as claimed in claim 1 wherein the disc iscomposed of sheet metal of a thickness which allows it to be easily bentto form different adjustable angles of inclination to the plane of thedisc.
 7. A microwave oven as claimed in claim 1 wherein said aperture islocated in a side wall of the cavity near the top wall and said stirreris mounted on a vertical shaft extending from said cavity top wall.
 8. Amicrowave oven as claimed in claim 1 wherein at least one diametricallyopposed pair of end segments are inclined in the same direction to andat the same angle to the plane of the disc.
 9. A microwave ovencomprising a substantially closed metallic cavity adapted to hold anenergy absorbing load, means for supplying microwave energy into theoven cavity, a field stirrer rotatably mounted within the cavity andcomprising a thin metal circular disc having a plurality of radial slotstherein to form radial fins arranged about the disc periphery with atleast some of the fins inclined in a direction at an angle to the planeof the disc, at least a part of said slots lying in the plane of thedisc to function as semiresonant elements, and means for rotating saiddisc to intercept and interact with the energy supplied to the ovencavity thereby to produce a cyclically repetitive distribution of energywithin the cavity which is substantially uniform in the region of thecavity to be occupied by the load.
 10. A microwave oven as claimed inclaim 9 wherein said fins are uniformly disposed about the discperiphery and at least some of which are inclined at different angles tothe plane of the disc.
 11. A microwave oven comprising a substantiallyclosed metallic cavity adapted to hold an energy absorbing load, meansfor supplying microwave energy into the oven cavity, a field stirrerrotatably mounted in the cavity and comprising a thin metal circulardisc having a plurality of radial slots symmetrically arranged about itsperiphery to form a plurality of segments with the outer ends of atleast some of said segments being bent out of the plane of the disc,said slots being dimensioned so as to form semiresonant elements towhich energy from said energy supplying means is coupled directly andthen is reradiated to the oven cavity, and means for rotating said discso as to intercept and interact with the energy from said energysupplying means and the energy within the fields in the oven cavitythereby to improve the energy distribution within said cavity.
 12. Amicrowave oven as claimed in claim 11 wherein the bent ends of at leastsome of said disc segments are inclined at different angles to the planeof the disc.
 13. A microwave oven as claimed in claim 12 wherein atleast one diametrically opposed pair of end segments are inclined in thesame direction to and at the same angle to the plane of the disc.
 14. Amicrowave oven as claimed in claim 13 wherein the bending angle of theend segments varies progressively about the disc periphery.
 15. Amicrowave oven as claimed in claim 12 wherein some of the disc endsegments are inclined upward from the plane of the disc and other onesof said end segments are inclined downward from the plane of the disc.16. A microwave oven as claimed in claim 11 wherein the width of theslots is relatively narrow with respect to the width of the segment areabetween adjacent slots in the plane of the disc.